A bulb is any plant that stores its complete life cycle in an underground storage structure. A corm is a swollen stem base that is modified into a mass of storage tissue; it does not have visible storage rings when cut in half. A tuber differs by not having a basal plant from which roots develop and not having a protective tunic covering. Rhizomes are horizontal underground stems that strike new roots out of their nodes, down into the soil, and that shoot new stems out of their nodes, up to the surface. This rhizome activity represents a form of plant reproduction.